XIV ZOANTIIAKIA ZOANTHIDEA 4O5 



be attached to foreign bodies. The genera Gemmaria and 

 Isanrtis inchide solitary forms. 



In the majority of the species of Zoanthids, however, a basal 

 encrusting stolon is formed, which may be thick and fleshy or 

 membranous, or may consist of a plexus of bands from which 

 several zooids rise and on which the new buds are formed. 



The tentacles are numerous, simple, usually short, and 

 arranged in one or two circles on the margin of the dise. 

 Most Zoantliidae are encrusted with sand, shell fragments, or 

 sponge spicules, but Zoanthus and Isaurus are naked. The 

 foreign particles that form the incrustation are firmly attached 

 to the ectoderm, and as a rule many of them sink down into 

 the mesogloea to give additional support to the body-wall. It 

 is the presence of so much incorporated sand that frequently 

 gives these Zoantharia such a very brittle character. The 

 stomodaeum usually exhibits a well-marked ventral siphonoglyph. 

 The mesenteries consist of a pair of complete ventral directives, 

 a pair of incomplete dorsal directives, while of tlie remaining 

 protocnemes the lateral mesenteries which are first and second 

 in the order of appearance are complete, the sixth is incomplete, 

 whereas the fifth is complete in the Macrocneminae and incom- 

 plete in the Brachycneminae. Duerden ^ has found in specimens 

 of three species that the arrangement of the mesenteries is 

 " brachycnemic " (the sixth protocneme imperfect) on one side 

 and " macrocnemic " (the sixth protocneme perfect) on the other. 

 The metacnemes appear in the spaces between the sixth 

 protocnemes and the ventral directives in unilateral pairs, of 

 which one becomes complete and tlie other always remains 

 incomplete (Fig. 163, 4, p. 368). 



The Zoanthidae are usually dioecious, but hermaphroditism 

 undoubtedly occurs in the genera Zoanthus and Isaurus. Little 

 is known of their development, but a larval form discovered by 

 Semper off the Cape of Good Hope, of cylindrical shape, with an 

 opening at each end and distinguished by a longitudinal band of 

 cilia running from one end to the other, is probably the larva 

 of a Zoanthid. It is commonly known as Semper's larva. Other 

 larvae provided with a ring of cilia have also been attributed to 

 this group. 



A great many Zoanthidae are epizoic in liabit. Thus several 



' Trans. Jloij. Hoc. l>uU. <'2) vi. 189S, ji. 331. 



